Gas turbine exhaust passage and damper system for same

ABSTRACT

A gas turbine exhaust passage is realized which may suppress the radiation of strong ultra low frequency noise to the outside without amplifying the turbulence of an high speed exhaust gas flow rate or a pressure pulsation generated in a gas turbine. At least a portion of a wall of a gas turbine exhaust passage ( 3, 5 ) is formed of an acoustically transmissive material ( 36, 56 ) for allowing a low frequency noise of several tens of Hz or less to pass therethrough sufficiently. Also, the acoustically transmissive material is made of one or more of a porous material, a porous heat insulating material, a mesh having a large flow resistance, cloth or film material. Further, the acoustically transmissive material is supported by a porous plate or a frame. In a case where an acoustically transmissive material is used only for an exhaust chimney ( 5 ), the exhaust chimney ( 5 ) is supported by a rack ( 11 ). Further, a soundproof panel ( 12 ) may be attached to the rack ( 11 ). Also, a damper provided at a branch portion between an exhaust gas boiler branched from the gas turbine exhaust passage and the gas turbine exhaust passage is formed of acoustically transmissive material for allowing a low frequency noise of several tens of Hz or less to pass therethrough sufficiently.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a gas turbine exhaust passageapplied to simple cycle gas turbines and combined cycle gas turbineshaving a bypass chimney or the like and to a damper system for same.

[0003] 2. Description of the Related Art

[0004]FIG. 8 shows a conventional simple cycle gas turbine exhaustpassage, and FIG. 9 shows a conventional combined cycle gas turbineexhaust passage. In FIG. 8, the exhaust passage is connected to a gasturbine body 1 through an exhaust diffuser 2 and is composed of anexhaust duct 3 provided with an internal exhaust silencer 4, and anexhaust chimney 5 connected to the exhaust duct 3. On the other hand, inFIG. 9, the exhaust passage is connected to the gas turbine body 1through the exhaust diffuser 2 and is composed of an exhaust duct 3provided with an internal exhaust silencer 4, and a bypass chimney 6connected to the exhaust duct 3.

[0005] The general cross-sectional structures of the walls of theabove-described conventional gas turbine exhaust passages are shown onan enlarged scale in FIGS. 8 and 9. In general, two types ofcross-sectional structures are used. The enlarged cross-sectionalstructures shown in portion D in FIG. 8 and portion F in FIG. 9 arethose used in the exhaust ducts 3 of the respective exhaust passages. Inthese cross-sectional structures, a heat insulating material 32 issandwiched between an inner plate 31 and an outer plate 33. Also, theenlarged cross-sectional structures shown in portion E in FIG. 8 andportion G in FIG. 9 are those used in the exhaust passage chimney 5 orthe bypass chimney 6 of the respective exhaust passages. In thesecross-sectional structures, a refractory member 52 such as refractorybricks is lined on the inside of the outer plate 53. There is no gaspermeability or acoustic transmissivity with either of these structures.

[0006] Also, as shown in FIG. 9, in the case of a combined cycle gasturbine exhaust passage, a boiler inlet duct 7 connected to the chimney10 through an exhaust gas boiler 8 is connected to the exhaust duct 3. Abypass damper 9 is provided at a joint portion between the boiler inletduct 7 and the exhaust duct 3. Also, as with the cross-sectionalstructure of each of the above-described walls, the bypass damper 9 hasa structure with which there is no air permeability or soundtransmissivity. The gas flow and the acoustic characteristics in theexhaust passage during the bypass operation are substantially the sameas those of the exhaust passage shown in FIG. 8.

[0007] In the gas turbine exhaust passage having the conventionalcross-sectional wall structure, pressure pulsation and high speedexhaust gas flow turbulence generated in a combustor of the gas turbineare amplified at a resonant frequency of the exhaust passage and anultra low frequency noise in the range of 10 to 30 Hz is emitted fromthe opening of the chimney. As a result, ultra low frequency noisetrouble occurs. Since portions other than the chimney opening areacoustically sealed due to the cross-sectional structure of the walls ofthe exhaust passage and the bypass damper, the damping of the soundfield is small and the resonance magnification is high. Hence, suchtrouble occurs.

SUMMARY OF THE INVENTION

[0008] Accordingly, an object of the present invention is to provide anexhaust passage which can maintain a good environment without causingthe ultra low frequency noise trouble.

[0009] According to the present invention, a gas turbine exhaust passagehaving a wall for defining an exhaust passage for discharging exhaustgas of a gas turbine is characterized in that at least a portion of thewall is formed of acoustically transmissive material for allowingsufficient low frequency noise of several tens of Hz or less to passtherethrough.

[0010] With this arrangement, since at least a portion of the wall isformed of an acoustically transmissive material, the damping effect ofan acoustic field within the exhaust passage is enhanced to make itpossible to reduce resonance magnification. Hence, it is possible tosuppress radiation of the strong ultra low frequency noise generated inthe gas turbine to the outside.

[0011] Also, the gas turbine exhaust passage according to the presentinvention is characterized in that the acoustically transmissivematerial is made of at least one material selected from the groupessentially consisting of a porous material, porous heat insulatingmaterial, mesh having a large flow resistance, and cloth or filmmaterial.

[0012] With this arrangement, since porous material, porous heatinsulating material, mesh having a large flow resistance, and cloth orfilm material may transmit sound sufficiently, it is possible to furthersuppress radiation of the strong ultra low frequency noise generated inthe gas turbine to the outside.

[0013] The gas turbine exhaust passage according to the presentinvention is further characterized in that the acoustically transmissivematerial is supported by a porous plate or frame.

[0014] With this arrangement, since the acoustically transmissivematerial is supported by the porous plate or frame, the acousticallytransmissive material may be used even if the transmissive material cannot maintain its shape.

[0015] Also, a damper system for a gas turbine exhaust passage,comprising a gas turbine exhaust passage for discharging the exhaust gasof a gas turbine, an exhaust gas boiler branched from the gas turbineexhaust passage, and a damper provided at a branch portion between theexhaust gas boiler and the gas turbine exhaust passage, is characterizedin that the damper is made of an acoustically transmissive material thatcan sufficiently transmit a low frequency noise of several tens of Hz orless.

[0016] With this arrangement, the damper provided at the branch portionbetween the gas turbine exhaust passage and the exhaust gas boiler ismade of the acoustically transmissive material. Accordingly, the soundwaves generated in the gas turbine are transmitted to the exhaust gasboiler through the damper. The sound waves transmitted to the exhaustgas boiler are absorbed by the exhaust gas boiler because the exhaustgas boiler has a large volume. Hence, it is thus possible to suppressradiation of the strong ultra low frequency noise from the gas turbineexhaust passage to the outside.

[0017] Also, according to the present invention, a gas turbine exhaustpassage having a wall for defining an exhaust passage for dischargingexhaust gas of a gas turbine, comprising an exhaust duct connected to agas turbine body through an exhaust diffuser and provided with aninternal exhaust silencer, and an exhaust chimney connected to theexhaust duct, is characterized in that at least a portion of the wall isformed of acoustically transmissive material for allowing low frequencynoise of several tens of Hz or less to pass therethrough sufficiently.

[0018] Also, according to the present invention, a damper system for agas turbine exhaust passage, comprising an exhaust duct connected to agas turbine body through an exhaust diffuser and provided with aninternal exhaust silencer, a bypass chimney connected to the exhaustduct, an exhaust gas boiler branched at a branch portion from theexhaust duct, and a damper provided between the exhaust gas boiler andthe exhaust duct, is characterized in that the damper is formed of anacoustically transmissive material for allowing low frequency noise ofseveral tens of Hz or less to pass therethrough sufficiently.

[0019] In any of the above cases, it is possible to suppress theradiation of the strong ultra low frequency noise to the outside.

[0020] Also, according to the present invention, a gas turbine exhaustpassage having a wall for defining the exhaust passage for dischargingexhaust gas of a gas turbine, comprising an exhaust duct provided withan internal exhaust silencer and an exhaust chimney connected to theexhaust duct, is characterized in that the wall of the exhaust duct hasa structure having no gas permeability and acoustic transmissivity, atleast a portion of the wall of the exhaust chimney is formed of anacoustically transmissive material for allowing sufficient low frequencynoise of several tens of Hz or less to pass therethrough, and a rack isprovided as surrounding and supporting the exhaust chimney.

[0021] In the above arrangement, since the wall of the exhaust chimneyhas a structure having acoustic transmissivity, an emission of the ultralow frequency noise can be restrained and transmission of noise from thewall of the exhaust duct can be prevented. Therefore, the risk of anaccident caused by spouting an exhaust gas can be eliminated, therebybeing capable of lightening and simplifying the wall structure of theexhaust chimney.

[0022] Further, the gas turbine exhaust passage, according to thepresent invention, is characterized in that at least a portion of theside and top of the frame formed the rack is provided with a soundproofpanel.

[0023] In the above arrangement, noise emitted from the exhaust passageis absorbed and reduced by the soundproof panel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In the accompanying drawings:

[0025]FIG. 1 is an illustration of a gas turbine exhaust passageaccording to a first embodiment of the present invention, wherein eachcircle indicated by a dashed line is an enlarged cross-sectional viewshowing portion A or B;

[0026]FIG. 2 is a graph showing a comparison of the power levels of thegas turbine exhaust passage according to the first embodiment and aconventional gas turbine exhaust passage;

[0027]FIG. 3 is an illustration of a gas turbine exhaust passageaccording to a second embodiment of the present invention, wherein acircle indicated by a dashed line is an enlarged cross-sectional view ofa portion C;

[0028]FIG. 4 is a graph showing a comparison of the power levels of thegas turbine exhaust passage according to the second embodiment and aconventional gas turbine exhaust passage;

[0029]FIG. 5 is an illustration of a gas turbine exhaust passageaccording to a third embodiment of the present invention, whereincircles indicated by a dashed line are enlarged cross--sectional viewsof portions D, E, and F, respectively;

[0030]FIG. 6 is a graph showing a comparison of the power levels of thegas turbine exhaust passage according to the third embodiment and aconventional gas turbine exhaust passage;

[0031]FIG. 7 is an illustration of a gas turbine exhaust passageaccording to a fourth embodiment of the present invention, wherein FIG.7A is a structural side view, and FIG. 7B is a plan view taken along theline L-L of FIG. 7A.

[0032]FIG. 8 is an illustration of a conventional simple cycle gasturbine exhaust passage, wherein circles indicated by dashed lines arean enlarged cross-sectional views of portion D or E; and

[0033]FIG. 9 is an illustration of a conventional combined cycle gasturbine exhaust passage, wherein circles indicated by dashed lines areenlarged cross-sectional views of portion F or G.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The present invention will be described in detail in conjunctionwith what is presently considered as preferred or typical embodimentsthereof by reference to the drawings.

[0035] In the following description, like reference characters designatelike or corresponding parts throughout the several views. Also in thefollowing description, it is to be understood that such term as “left”,“right”, “top”, “bottom” and the like are words of convenience and arenot to be construed as limiting terms.

[0036] A gas turbine exhaust passage in accordance with a firstembodiment of the present invention will now be described with referenceto FIG. 1. Further, the present embodiment is directed to the case asimple cycle gas turbine.

[0037] The exhaust passage of the gas turbine shown in FIG. 1 iscomposed of an exhaust duct 3 provided with an internal exhaust silencer4 and connected to a gas turbine body 1 through an exhaust diffuser 2,and an exhaust chimney 5 connected to the exhaust duct 3. Enlargedcross-sectional views of cross-sectional structures of walls of theexhaust duct 3 and the exhaust chimney 5 are shown in portion A andportion B, respectively, in circles indicated by dashed lines in FIG. 1.The cross-sectional structure of the wall of the exhaust duct 3 is asandwich structure formed by sandwiching a porous heat insulatingmaterial 36 with a porous inner plate 35 and a porous outer plate 37.Also, the cross-sectional structure of the wall of the exhaust chimney 5is a structure obtained by lining a porous material 56 on a porous outerplate 57.

[0038] The gas turbine exhaust passage provided with the wall structuresshown on an enlarged scale in portion A and portion B of FIG. 1 is anacoustically transmissive structure which allows the low frequency noiseof several tens of Hz or less to permeate therethrough. As specificmaterials for forming this structure, glass wool or rock wool may beemployed as the porous heat insulating material 36, and a cloth, a metalmesh, a sound-absorbing panel, a ceramic, a foamed concrete and the likehaving a communication hole may be employed as the porous material 56.Also, with respect to the porous outer plate 57, if the porous material56 is able to maintain its own shape, the mounting frame may be usedalone.

[0039] Further, in the embodiment, the cross-sectional structure of thewall of the exhaust duct 3 is that of the sandwich structure, as shownin the enlarged view in portion A of FIG. 1; and the cross-sectionalstructure of the wall of the exhaust chimney 5 is that of the internallylined structure, as shown in the enlarged view in portion B of FIG. 1.However, these structures may be substituted for one another. Also, theexhaust duct 3 and the exhaust chimney 5, as a whole, may have thesandwich structure or the internally lined structure. Furthermore, onlythe exhaust duct 3 or the chimney 5 singly or rather portions of eachmay have the acoustically transmissive structure.

[0040] In the gas turbine exhaust passage according to this embodimentof the present invention, since the exhaust passage is formed with theacoustically transmissive structure in which sound may readily betransmitted as described above, unlike the conventional exhaust passagein which portions other than the opening of the exhaust chimney areacoustically sealed, there is no fear that the pressure pulsation andthe high speed exhaust gas flow turbulence generated in a combustor ofthe gas turbine will be amplified at a resonance frequency of theexhaust passage. The reason for this is that the above-describedacoustic transmissive structure makes it possible to enhance the dampingeffect of the acoustic field within the exhaust passage, and it ispossible to considerably lower the resonance magnification. As a result,it is possible to suppress the strong ultra low frequency noise of 10 to30 Hz emitted from the chimney opening and to avoid the low frequencynoise trouble.

[0041]FIG. 2 shows the measurement results of the PWL (power level)radiated from the exhaust chimneys of the gas turbine exhaust passageaccording to this embodiment of the present invention and those of aconventional gas turbine exhaust passage, respectively. Here, the solidline represents the measurement results of the conventional exhaustpassage and the dotted line represents the measurement results of thepresent embodiment. It has been found that, compared to the conventionalexhaust passage, the PWL of the present embodiment is lowered by 10 dBor more at each peak spectrum, and the exhaust passage according to thepresent embodiment is very effective. Incidentally, it is more effectiveto make the sound transmissive structure for the exhaust passage as awhole. However, it is understood that only the countermeasure for theexhaust chimney 5 is also sufficiently effective.

[0042] The gas turbine exhaust passage according to a second embodimentof the present invention will now be described with reference to FIG. 3.Incidentally, this embodiment is for the case of the combined cycle gasturbine with the bypass chimney.

[0043] In the gas turbine exhaust passage according to the secondembodiment of the present invention shown in FIG. 3, a boiler inlet duct7 branched from the exhaust duct 3 and connected to a chimney 10 throughan exhaust gas boiler 8 is provided in addition to the gas turbineexhaust passage of the above-described simple cycle. A bypass damper 9is provided at a joint portion between the boiler inlet duct 7 and theexhaust duct 3. An enlarged view of portion C is shown by a dashed linecircled portion of FIG. 3. The bypass damper 9 has a sandwichingcross-sectional structure formed by placing the porous material 92between two porous plates 91.

[0044] The porous material 92 placed between the two porous plates 91 ismade of metal mesh or cloth and allows the sound to pass therethrough,but becomes very resistant to the flow of exhaust gas. Theabove-described bypass damper 9 has a structure in which the exhaust gasis not allowed to pass therethrough, but sound may.

[0045] In the embodiment of the present invention, since the bypassdamper 9 is acoustically transmissive, the sound wave generated in thegas turbine permeates through the bypass damper to be transmitted to theexhaust boiler 8 side, and the sound radiated from the bypass chimney 6is decreased. Also, because the volume of the exhaust gas boiler 8 islarge, the ultra low frequency noise transmitted to the exhaust gasboiler side is attenuated by the exhaust gas boiler. As a result, it ispossible to prevent radiation of the strong ultra low frequency noisefrom both the bypass chimney 6 and the boiler chimney 10.

[0046]FIG. 4 shows the measurement results of the PWL radiated from thebypass chimneys and the boiler chimneys of the gas turbine exhaustpassage according to this embodiment of the present invention and theconventional gas turbine exhaust passage, respectively. Here, the solidline represents the measurement results of the conventional exhaustpassage and the dotted line represents the measurement results of thepresent embodiment. It has been found that, comparison with theconventional exhaust passage, in the present embodiment the peakspectrum of the ultra low frequency noise is considerably lowered.

[0047] A gas turbine exhaust passage, according to a third embodiment ofthe present invention, will be described with reference to FIG. 5. Inthis embodiment, a simple cycle gas turbine is employed, and aframe-like supporting is provided around the gas turbine exhaustpassage.

[0048] The gas turbine exhaust passage shown in FIG. 5, according to thethird embodiment of the present invention, comprises an exhaust duct 3connected to a gas turbine body 1 by way of an exhaust diffuser 2 andprovided with an exhaust silencer 4 therein, a deformed duct 5 aconnected to the exhaust duct 3, and a exhaust chimney 5, as shown inthe structural side view. In the gas turbine exhaust passage, a partialor an entire cross-sectional structure of the wall of the exhaustchimney 5 is a structure such that a porous material 82 winds around aporous outer plate 81, as shown in an enlarged cross-sectional view ofportion D indicated by a circle with a dashed line. Further, the exhaustchimney 5 is supported by the rack 11.

[0049] A cross-sectional structure of the wall of the exhaust chimney 5in portion E above the rack 11 comprises an outer plate 41 and a thermalinsulating material 42. Also, the cross-sectional structures of thewalls of the deformed duct 5 a and the exhaust duct 3 are, as shown inan enlarged view of portion F, a sandwich structure formed bysandwiching a heat insulating material 72 with an outer plate 71 and aninner plate 73. The structure of the above-mentioned portions E and F donot have air permeability or sound transmissivity.

[0050] The cross-sectional structure of the wall shown in the enlargeddiagram of portion D in FIG. 5 is acoustically transmissive to allow thelow frequency noise of several tens of Hz or less to pass therethrough.That is, although the structure allows the noise to pass therethrough,it has a sufficient pressure loss to prevent the exhaust gas or the airfrom passing therethrough. As specific materials for forming thisstructure, a rock wool, a foamed concrete, a ceramic, a sound-absorbingpanel, a metal mesh, a cloth, a wire mesh or the like may be employed asthe porous material 82. Also, with respect to the porous outer plate 81,structure materials with which the porous material 82 is able tomaintain its own shape, such as a porous plate, may be employed.

[0051] The wall having the acoustically transmissive structure shown inthe present figure is up to portion D, that is, the upper portion of therack 11. Since the rack 11 supports an acoustically transmissivestructure body, and holds a structure body shown as portion E above theupper portion of the rack 11, it is sufficient if the acousticallytransmissive structure body has enough strength to support itself.Further, with respect to the porous outer plate 81 and the porousmaterial 82, as shown in the enlarged view of portion D, the inner andouter structures may be substituted for one another. Also, the entirewall of the exhaust chimney 5 may be substituted for the structure shownin the enlarged view of portion D. However, in this case, it ispreferable that the rack 11 is extended to the top of the exhaustchimney 5 to support the exhaust chimney 5.

[0052] Also in the present embodiment, since the exhaust chimney 5employs the acoustically transmissive structure with which a sound canreadily pass therethrough, the gas turbine exhaust passage of theinvention can enhance a damping effect in an acoustic field with theexhaust passage, and it is possible to considerably lower resonancemagnification. As a result, it becomes possible to suppress the emissionof strong ultra low frequency noise to the outside. Also, the exhaustduct 3 is left unchanged from a conventional structure, thereby beingcapable of preventing the transmission of audible sound and accidentssuch as a scald of man caused by exhaust gas discharging.

[0053] Further, the gas turbine exhaust passage of the invention has therack 11, and therefore the acoustically transmissive structure portionof the exhaust chimney 5 may only have a strength to the extent that cansupport itself, which enables the structure of the wall to be lightenand simplified.

[0054]FIG. 6 shows the measurement results of the sound volumes of thesound waves emitted from the exhaust passage according to the thirdembodiment of the present invention and those of a conventional gasturbine exhaust passage, respectively. In this case, a solid linerepresents a measurement result of the conventional gas turbine exhaustpassage and a dotted line represents a measurement result of the gasturbine exhaust passage of the present embodiment. It has been foundthat, comparison with the conventional exhaust passage, in the presentembodiment, is lowered by 10 dB or more at each peak spectrum, andtherefore, the gas turbine exhaust passage according to the presentembodiment is considerably effective.

[0055] The gas turbine exhaust passage, according to a fourth embodimentof the present invention, will be described with reference to FIG. 7. Inthe present embodiment as well as in the third embodiment, a simplecycle gas turbine is employed, and a supporting rack is provided aroundthe gas turbine exhaust passage.

[0056] The arrangements of the exhaust passage and the rack 11 arebasically the same as those in the third embodiment shown in FIG. 5.However, as shown in the structural side view, FIG. 7A, and in FIG. 7Bwhich is a plan view taken along the line L-L in FIG. 7A, an entire or aportion of the side and top of the rack 11 is provided with a soundproofpanel 12.

[0057] As the soundproof panel 12, materials which cut off a sound andhas a structure to absorb a sound, such as ALC (Autoclaved LightweightConcrete) plate, a sound insulating plate, a panel formed by attaching asound-absorbing panel to the sound insulation plate or the like, can beemployed.

[0058] In the gas turbine exhaust passage according to the presentembodiment, an effect of reducing the emission of the ultra lowfrequency noise may be obtained by the same operation as in the gasturbine exhaust passage according to the third embodiment has. Further,the gas turbine exhaust passage according to the present embodiment hasthe same structural advantages as the gas turbine exhaust passageaccording to the third embodiment has. Also, since the soundproof panel12 is attached by way of the rack 11 surrounding the exhaust passage,there is an effect of further absorbing and reducing noise emitted fromthe exhaust passage.

[0059] Various details of the invention may be changed without departingfrom its spirit or its scope. Furthermore, the foregoing description ofthe embodiments according to the present invention are provided for thepurpose of illustration only, and not for the purpose of limiting theinvention as defined by the appended claims and their equivalents.

What is claimed is:
 1. A gas turbine exhaust passage having a wall fordefining an exhaust passage for discharging exhaust gas of a gasturbine, wherein at least a portion of said wall is formed of anacoustically transmissive material for allowing a low frequency noise ofseveral tens of Hz or less to pass therethrough sufficiently.
 2. The gasturbine exhaust passage according to claim 1, wherein said acousticallytransmissive material is made of at least one material selected from thegroup essentially consisting of a porous material, porous heatinsulating material, mesh having a large flow resistance, cloth and filmmaterial.
 3. The gas turbine exhaust passage according to claim 2,wherein the acoustically transmissive material is supported by a porousplate or frame.
 4. A damper system for a gas turbine exhaust passage,comprising a gas turbine exhaust passage for discharging exhaust gas ofa gas turbine, an exhaust boiler branched from said gas turbine exhaustpassage, and a damper provided at a branch portion between said exhaustboiler and said gas turbine exhaust passage, wherein said damper is madeof an acoustically transmissive material that may sufficiently transmita low frequency noise of several tens of Hz or less.
 5. The dampersystem for a gas turbine exhaust passage, according to claim 4, whereinsaid acoustically transmissive material is made of at least one materialselected from the group essentially consisting of a porous material,porous heat insulating material, mesh having a large flow resistance,cloth and film material.
 6. The damper system for a gas turbine exhaustpassage, according to claim 5, wherein the acoustically transmissivematerial is supported by a porous plate or frame.
 7. A gas turbineexhaust passage having a wall for defining an exhaust passage fordischarging exhaust gas of a gas turbine, comprising an exhaust ductconnected to a gas turbine body through an exhaust diffuser and providedwith an internal exhaust silencer, and an exhaust chimney connected tosaid exhaust duct, wherein at least a portion of said wall is formed ofan acoustically transmissive material for allowing a low frequency noiseof several tens of Hz or less to pass therethrough sufficiently.
 8. Adamper system for a gas turbine exhaust passage, comprising an exhaustduct connected to a gas turbine body through an exhaust diffuser andprovided with an internal exhaust silencer, a bypass chimney connectedto said exhaust duct, an exhaust gas boiler branched at a branch portionfrom said exhaust duct, and a damper provided between said exhaust gasboiler and said exhaust duct, wherein said damper is formed of anacoustically transmissive material for allowing a low frequency noise ofseveral tens of Hz or less to pass therethrough sufficiently.
 9. A gasturbine exhaust passage having a wall for defining an exhaust passagefor discharging exhaust gas of a gas turbine, comprising an exhaust ductprovided with an exhaust silencer therein and an exhaust passageconnected to the exhaust duct, wherein the wall of the exhaust duct hasa structure having no gas permeability and acoustic transmissivity, atleast a portion of the wall of the exhaust chimney is formed of anacoustically transmissive material for allowing a low frequency noise ofseveral tens of Hz or less to pass therethrough sufficiently, and a rackis provided to surround and support said exhaust chimney.
 10. The gasturbine exhaust passage, according to claim 9, wherein a soundproofpanel is attached to at least a portion of the side and top of a frameformed said rack.